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Craford bags innovation prize for LED development

Veteran LED technologist and Lumileds CTO George Craford wins an Economist innovation award, and thinks that a team from the company has now identified the root cause behind a long-standing problem in high-power InGaN LEDs.

George Craford, one of the world s best-known names in LED technology development, has won the 2007 Economist Innovation award for Energy and the Environment.

Currently CTO at leading LED company Philips Lumileds, Craford has been working in the field for the past forty years, and was previously at Monsanto and Hewlett-Packard.

Tom Standage from the Economist said that pioneering work by Craford and Roland Haitz meant that "the proportion of human energy consumption attributable to lighting will finally go down".

Receiving the award at London s Science Museum, Craford said, "It is wonderful to think that my work and that of my colleagues has helped to create a practical replacement to conventional lighting, that is both energy-efficient, and that can significantly reduce the impact of lighting on the environment."

But Craford acknowledges that there is still plenty more work to be done on the innovation front before LEDs take over in any meaningful way for general lighting.

Speaking to compoundsemiconductor.net while in London, Craford stressed the need to further improve InGaN-based LEDs to ensure that solid-state lighting fulfills its commercial potential.

"There s no longer any question that LEDs will take over," Craford said.

The big question that does remain, however, is exactly how technologists will crack the problem of reduced IQE in InGaN emitters when they are driven at high current densities - although Craford believes that a Philips Lumileds team has now identified the key to that particular issue.

According to the team s recent photoluminescence study of InGaN material, it is the phenomenon of Auger recombination that causes this so-called "droop" in IQE.

Previously, this problem has been attributed to a number of other physical mechanisms, such as high plasma temperatures, a high density of threading dislocations, or pockets of InGaN material with a high InN composition.

Auger recombination is a non-radiative decay mechanism that occurs when three particles interact. As such, it is inherently weak, and is usually only of any significance when the density of charge carriers in a semiconducting material is extremely high.

But, when Auger recombination does occur, energy that would otherwise be emitted in the form of light is instead lost thermally. In an LED, that would mean reduced IQE and excess heat.

If Craford s colleagues are right, and Auger recombination is the root cause of this efficiency "droop" in InGaN LEDs, it may help to solve what has been a long-standing problem in the development of very-high-brightness blue, green and white emitters.

Craford himself believes that the Auger theory is the correct one. His hunch is that increasing the recombination volume in LED designs, such as by depositing thicker structures, will reduce the carrier density and therefore the Auger-related loss.

His hope is that a combination of this and other evolutionary developments could now lead to devices suitable for general lighting applications, perhaps even to a 1 mm-scale chip capable of emitting 500 lumens.

"I hope that we don t have to do something radically different," said the CTO.

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